Appearance of a hyposialylated transferrin fraction in the plasma during chronic alcohol exposure is a well-known phenomenon, and it represents the best available marker of chronic alcohol consumption. The mechanisms of its appearance are still not well understood and are extremely complex, involving biosynthesis and catabolism alterations, although the only structural abnormality described corresponds to the loss of an entire glycan chain. We analyzed and compared the oligosaccharides present on the different isoforms of purified transferrin isolated from control and patients with severe alcohol abuse by fluorescent carbohydrate electrophoresis and matrix-assisted laser desorption ionization mass spectrometry. Our data indicate that the major modification observed is the loss of an entire oligosaccharide chain; we also demonstrate that there is a modification of terminal sialylation. Carbohydrate-deficient transferrin (CDT) is the result of multiple alterations of glycosylation. These results give a partial explanation to the poor sensitivity of the measurement of CDT and its controversial use as a marker of chronic alcohol consumption.